Efficient Type Three Secretion (T3S) by Gram-negative bacteria requires adhesion to the targeted host cell. We have recently determined that the Yersinia pestis adhesin Ail facilitates T3S of Yersinia outer proteins (Yops) via binding to host fibronectin. Yop delivery is required for plague infection and mutants that lack Yops are completely avirulent. A ?ail mutant has poor Yop delivery in vitro and is attenuated in vivo (>3000-fold increase in LD50). Thus, Ail appears to be a prominent adhesin for Yop delivery in vivo. This makes Ail an important potential target for Y. pestis vaccine development and anti-plague therapies. We hypothesize that Ail in conjunction with fibronectin interacts with host cells to allow efficient Yop delivery, possibly by stimulating receptor-initiated cell signaling required for Yop delivery. We further hypothesize that Fn acts as a bridge between Ail-expressing bacteria and 21 integrins on host cells (receptors for Fn). ?1 integrin signaling has recently been shown to be important for invasin-mediated Yop delivery by the closely related pathogen, Y. pseudotuberculosis. In this proposal, we will define the regions of Ail that interact with Fn and address the effects of Ail binding to Fn and subsequent engagement of ?1 integrins. By understanding this critical step in the Yop delivery process by a key adhesin of Y. pestis, we will significantly expand our knowledge of the role of adhesion in Yop delivery in particular and T3S in general. These studies will be broadly applicable to other bacterial pathogens that utilize a T3S mechanism for toxin delivery. Furthermore by defining a crucial interaction mechanism between Y. pestis and host cells, we may identify targets for therapeutics that can be used to treat plague, a rapidly fatal disease and bioterrorism threat. The Aims of this proposal are: Aim 1: Determine the residues of Ail required for host cell binding and Yop delivery Aim 2: Characterize Ail binding to host cell fibronectin and the role of 21 integrins in Ail-mediated Yop delivery PUBLIC HEALTH RELEVANCE: Yersinia pestis causes the rapidly fatal infectious disease plague, making Y. pestis a bioterrorism threat. This project is aimed at furthering our understanding of the interaction of the Y. pestis surface protein Ail with host cells. This critical interaction is required for plague virulence and characterizing this interaction may lead to development of anti-plague therapeutics and/or an effective plague vaccine.